1. Field of The Invention
The present invention relates to an improved thin film of oxide superconductor and a process for preparing the same, more particularly, an improved thin film of Cu-containing compound oxide such as YBCO having controlled crystal orientation deposited on a substrate of silicon wafer and a process for preparing the same.
2. Description of the Related Art
Superconducting phenomenon has been explained as unique phenomenon which can be observed only at extremely low temperatures which can be realized with liquid helium. However, a new type superconducting compound oxide of (La, Sr).sub.2 CuO.sub.4 which exhibits the superconductivity at 30 K. was reported in 1986 by Bednorz and Muller (Z. Phys. B64, 1986, 189). Then, another superconducting material of YBa.sub.2 Cu.sub.3 O.sub.y having higher critical temperature of about 90 K. was reported in 1987 by C. W. Chu et al. (Physical Review letters, Vol. 58, No. 9, p. 908) and then Maeda et al. reported so-called bismuth type compound oxide superconductor having the critical temperature of about 100 K. in 1988 (Japanese Journal of Applied Physics. Vol. 27, No. 2, p. 1209 to 1210).
These superconducting compound oxides are expected to be applied to actual uses since the superconducting phenomenon can be realized with relatively cheap cryogen of liquid nitrogen. In early stage of development, these superconducting compound oxides were prepared in a sintered body form by solid reaction and now, it has become possible to prepare thin films of high quality from these superconducting compound oxide materials.
When thin films of oxide superconductor are used in electronics, it is requested to prepare their thin film whose crystals are well-oriented. In fact, oxide superconductors possess high anisotropy in their superconducting properties and hence it is necessary to control crystal orientation at film-forming stage so as to adjust to applications. In the thin films of oxide superconductors, higher current flows to a direction which is perpendicular to c-axis. For example, when the thin film of oxide superconductor is used as superconducting wiring lines in a circuit, it is required to pass superconducting current along a direction which is in parallel with a surface of substrate. In this case, the c-axis of crystals of the thin film must be perpendicular with respect to the surface of substrate. In other words, (110) oriented thin films of oxide superconductor are required in these applications. In another application such as layered SIS (superconductor-insulator-superconductor) type device, it is required to pass superconducting current along a direction which is perpendicular to the surface of substrate. In this case, the c-axis of crystals of the thin film prepared must be perpendicular to the surface of substrate. In other words, (100) oriented thin film or (110) oriented thin films of oxide superconductor is required in these applications.
Importance of selection of deposition plane in substrate or optimization of deposition conditions, particularly substrate temperature for realizing a desired crystal orientation in a thin film of oxide superconductor was reported by many papers including Enomoto et al., Japanese Journal of Applied Physics, Vol. 26, No. 7, July 1987, pp.L1248-L1250, Asano et al. in Japanese Journal of Applied Physics, Vol. 28, No. 6, June 1989, pp.L981-L983 or the like.
It is also known that each of oxide superconductors possesses a complicated crystal structure and hence a desired crystal orientation can be realized only under a special deposition condition. In fact, the substrate on which a thin film of oxide superconductor is deposited must be selected from such substrates that have well-matched lattice constants with oxide superconductors and do not diffuse or migrate into the superconductor. From this point of view, MgO (100) single crystal or SrTiO.sub.3 (100) or (110) single crystal are used as the substrate.
Use of these substrates, however, has a demerit in industrial applications of oxide superconductors because they are costly material and are not produced in mass production. Still more, since diameters of these oxide single crystal substrates are limited, it is impossible to produce a thin film of oxide superconductor having a large area. It is also proposed to used, as substrate, a silicon single crystal substrate (Si wafer) which is less expensive and is available stably on market so as to prepare a thin film of oxide superconductor thereon.
However, it is known that, when a thin film of oxide superconductor is deposited on the silicon single crystal substrate, superconducting properties of the deposited thin film are seriously deteriorated or lost due to chemical reaction between oxide of which the superconducting thin film is made and silicon of which the substrate is made.
In order to solve this problem, it is proposed to interpose a buffer layer between the substrate and the thin film of oxide superconductor so as to prevent the diffusion. However, all of known buffer layers are not satisfactory to control or adjust crystal orientation of a thin film of oxide superconductor deposited on a silicon wafer.
Therefore, an object of the present invention is to solve the problem and to provide a process which permits to prepare a thin film of oxide superconductor having a desired crystal orientation on a substrate of silicon wafer.